Thermoplastic polyester moulding compositions

ABSTRACT

METHOD FOR MAKING SHAPED ARTICLES BY INJECTION MOULDING POLYESTER COMPOSITIONS COMPRISING AROMATIC DICARBOXYLIC ACIDS, SATURATED ALIPHATIC OR CYCLOALIPHATIC DIOLS AND ALCOHOLS HAVING MORE THAN TWO PRIMARY HYDROZYL GROUPS. THE SHAPED ARTICLES DO NOT SHOW FLASH FORMATION.

United States Patent US. Cl. 260873 9 Claims ABSTRACT OF THE DISCLOSURE Method for making shaped articles by injection moulding polyester compositions comprising aromatic dicarboxylic acids, saturated aliphatic or cycloaliphatic diols and alcohols having more than two primary hydroxyl groups. The shaped articles do not show flash formation.

The present invention relates to thermoplastic polyester moulding compositions which do not show flash formation when injection moulded under conventional conditions.

It has been proposed to process linear, saturated polyesters cf aromatic dicarboxylic acids by injection moulding. Injection moulding of polyethylene terephthalate has gained industrial importance. Injection moulded articles made of polyethylene terephthalate have, however, the drawback that after removal from the mould their dimensional stability is not maintained owing to after-crystallization, especially at high temperatures. It has repeatedly been proposed, therefore, to process polyethylene terephthalate with additions of inorganic or organic substances promoting crystallization.

In British Pat. No. 1,104,089 it has been described, for

example, that inorganic substances that are insoluble in the polyester melt, such as calcium carbonate, calcium sulfate or titanium dioxide and have a particle size of less than 2 microns accelerate the crystallization of the polyester composition. The inorganic substances are added to the polyesters in an amount of from 0.05 to 0.5% by weight. The addition ensures that at a sufficiently high temperature of the mould the polycondensation product reaches an optimum degree of crystallization during moulding so that alterations of shape and dimensions by after-crystallization are practically excluded.

. Still further, it has been proposed to incorporate into fiber-forming polyesters polyols having more than two hydroxyl groups. By this step the period of time required to obtain high molecular weights is shortened whereby the-mechanical properties of the fibers are improved. For the manufacture of films and coatings polyesters are being used which are produced from terephthalic acid, ethylene glycol and at least trihydric alcohol. Films made from polyesters of this type are still flexible and not crystalline after heating for 1 hour at 220 C.

Moulded articles made from polyester moulding compositions are predominantly used for industrial purposes, for example for the manufacture of gear wheels, pivot bearings and disk cams. They have to meet high standards with regard to their mechanical properties and, moreover, they should have an especially good surface quality, fill the mould completely, have a high dimensional stability and be free from flash formation. Although the conditions of injection moulding can be varied within wide limits, it is very diflicult to avoid flash formation with injection moulded polyethylene terephthalate. The method usually employed in injection moulding, i.e. to regulate the pressure during dwell time in such a manner that the mould "ing compositions.

is completely filled and the moulded articles remain free from flash formation, is often unsatisfactory. With a complete mould charge it is, therefore, necessary in most cases to subject the moulded article to a mechanical after-treatment after removal from the mould.

-It has now been found that polyester moulding compositions comprising (a) aromatic dicarboxylic acids and optionally small amounts of aliphatic dicarboxylic acids,

(b) saturated aliphatic or cycloaliphatic diols, and

(c) alcohols having more than two primary hydroxyl groups in an amount of from 0.01 to 2% by weight, preferably 0.03 to 0.5% by weight, calculated on the weight of the polyester,

do not show flash formation when injection moulded articles are made therefrom.

The moulding compositions according to the invention contain polyesters modified with small amounts of polyols which cause the formation of chain branchings.

As dicarboxylic acid in the polyester moulding compositions of the invention terephthalic acid is suitable in the first place. In addition to terephthalic acid units the polyester may contain up to 5 mole percent of other aromatic or aliphatic dicarboxylic acids as structural units, for example units of isophthalic acid, diphenyl-4,4-dicarboxyl1c acid, naphthalene-2,6-dicarboxylic acid or adipic acid.

As diol component ethylene glycol is especially suitable. It is likewise possible, however, to use 1,4-bis-hydroxymcthylcyclohexane. Besides ethylene glycol or 1,4- bis-hydroxymethylcyclohexane the diol component of the polyester may contain up to 10 mole percent of other allphatic diols, for example 2,2-dimethyl-propaned1ol-(1,3) or butanediol(l,4).

Suitable alcohols having more than two primary hydroxyl groups, which are designated as polyols, are, for. example: 1,1,1-tris-hyd.roxymethyl-ethane, 1,1,l,1-tris:hydroxymethyl-propane, pentaerythritol, dipentaerythrltol, 1,1,l,6,6,6-hexakis-hydroxymethyl-hexane l,1,3,3-tetrak1shydroxymethyl-cyclopentane and 1,2,4-tris-hydroxymethylbenzene. The polyols are used either individually or in admixture with one another.

The polyesters are prepared in known manner, for example by reacting diesters of the dicarboxylic acids and lower aliphatic alcohols with the diol and the polyol. The polyol can be added at dilferent stages of the manufacturing process of the polyester. It may be reacted with the diester of the dicarboxylic acid together with the diol.

.Alternatively, it maybe added to the reaction mixture after termination of the ester interchange reaction, whereupon the polyester is prepared by polycondensation in known manner.

' Ester interchange reaction and polycondensation are accelerated in knownmanner by adding small amounts of catalysts.

The thermoplastic moulding compositions according to the invention contain polyesters having a reduced specific viscosity in the range of from 0.6 to 2.4 dl/g., preferably 1.0 to 1.7 dl/g., measured with a 1% solution in a mixture of phenol and tetrachloroethane 60:40 at 25 C. To produce polyesters having a high reduced specific viscosity, the polyesters obtained by polycondensation in the melt are subjected to a post-condensation in the solid phase in known manner:

The polyol-modified polyesters may also be components of thermoplastic moulding compositions containing, besides the polyester, up to 20% by weight, preferably up to 10% by weight, of polymers, for example copolymers of ethylene and acrylic acid esters or of butadiene and styrene, which improve the impact strength of the mould It is suitable to add to the polyester a crystallization promoting agent, for example an inorganic nucleating agent such as kaolin, talc, or an alkaline earth metal carbonate. The inorganic nucleating agents should preferably phenol and tetrachloroethane at 25 C.) was obtained.

The granulated polyester was dried for hours at 180 C. under reduced pressure in an eccentric tumbling drier, subjected to rotation at room temperature together with have a particle size below 2 microns. It is likewise pos- 5 0.4% by weight of powdery aluminum silicate (47% sible to add to the polyesters organic crystallization SiO 38% A1 0 75% having a particle size of less than promoting agents, for example ionic copolymers of ethyl- 2 microns), the mixture was homogenized in an extruder, ene and acrylic acid, containing sodium ions as cations. granulated and then dried for 2 hours at 100 C. under a The crystallization promoting substances can be incorpressure of 0.4 mm. of mercury in an eccentric tumbling porated into the polyester composition in various ways. drier. The granules were then subjected to an after-con- The nucleating agent and the polyester can be mixed, for densation in the solid phase for 6 hours at 240 C. under example, by intense stirring in the molten state. It is ad- 0.4 mm. of mercury. The aftercondensed polyester had vantageous to mix as uniform as possible the granulated a reduced specific viscosity of 1.52 dl/ g. The granules or powdered polyester with the powder of the crystallizawere coated with 0.2% by weight of polyethylene wax. tion promoting agent, to melt the mixture in the extruder Sheets of dimensions 60 x 60 x 2 mm. were injection and to granulate it after cooling. moulded with the polyester composition at a temperature In order to obtain unobjectionable injection moulded of the mould of 150 C. The sheets were injection articles the polyester moulding composition should con moulded under different pressures, the pressure during tain as little moisture as possible, preferably at most dwell time being varied from 70 to 140 C. In each case 0.01% by weight. For this purpose the granulated poly the dwell time was 15 seconds. It was found that up to a ester moulding composition may be coated with a layer pressure of 130 atmospheres gauge no flashes were of an inert hydrophobic substance, for example a wax formed. or paraflin. (b) In a comparative experiment a polyester was pre- To produce shaped articles having a satisfactory degree pared as described above with the exception that no 1,1,1- of crystallization the temperature of the mould should tris-hydroxymethyl-propane was added prior to polyconbe sufficiently above the second order transition temperadensation. When the composition was injection moulded ture of the polyester. In the case of polyester moulding noticeable flash formation was already observed under a compositions on the basis of a modified polyethylene terpressure during dwell time of 70 atmospheres gauge, ephthalate mould temperatures in the range of from 120 which flash formation considerably increased with increasto 150 C. are preferred. ing pressure and was very pronounced under a pressure The following examples serve to illustrate the invention above 100 atmospheres gauge. but they are not intended to limit it thereto. EXAMPLE 2 EXAMPLE 1 A modified polyethylene terephthalate was prepared as (a) A mixture of kilograms of terephthalic acid 35 i i l contalnlllgr 8 0? by Weightf of dimethyl ester, 27 kilograms of ethylene glycol and 8.8 g i E g .1% e 5 yester was 5 grams of manganese acetate (tetrahydrate) was heated to i i i i l th e resulare summarfie 175 C. whilst stirring. During the course of 4 hours the O g a i 15 5 3 25 2 temperature was raised to 230 C. and the mixture was 4 j g g 0 mm W j 1 1 1 s f 130 o1- stirred until no more methanol distilled over. 52.5 grams 0 g: 3 er a pressure unng We me o atmosof 1,1,1-tris-hydroxymethyl-propane were then added. The p EXAMPLES melt was stirred for 15 minutes. Next 2.34 grams of phosphorous acid were added to the melt and, aftenstirring Two difi d ol th l terephthalates were 15 mlnqtes, grams of germfimum Phosphlte were pared as described in Example 1 which were modified added During the course of 150 minutes the temperature ith (3) 0,15% by Weight and (4) 0.10% by weight of was ralsed to 275 and P reduced to pentaerythritol. The polyesters were further treated as of Y- The melt was surfed for hours under these described in Example 1. The results with respect to the Condltlons temperature f p f A C010r16SS charge of the mould and flash formation when the compolyester hav ng a reduced specific vlscosity of 0.88 dl/g. positions were injection moulded are summarized in the (measured with a 1% solution in a :40 mixture of following table.

TABLE Mould charge and flash formation Speelfie viscosity under varying pressures during dwell time (atmospheres gauge) Before After Example Polyethylene terephthalate (1) injection injection Mould No. modified Withmoulding moulding Pressure charge (2) Flash (3) Mc- MC+ 1(a) 0.15% by weight 1,1,1-tris-hydroxy- 1. 52 1. 43 MC+ methyl-propane. 0+ MC+ 70 MC+ 90 MC+ 1(b) Nonmodified 1.41 1.35 110 MC+ 130 MC+ 140 MC+ 70 MC+ 90 MC+ 2 0.10% by wei ht 1,1,1-tris-hydroXy- 1.42 1. 34 110 MC+ propane. 130 MC+ 140 MC+ 70 MC+ 90 MC+ 3 0.15% by weight pentaerythritol. 1.50 1. 42 110 MC+ 130 MC+ 140 MC+ 70 MC+ 90 MC+ 4 0.1% by weight pentaerythritol 1.48 1.40 110 MC+ 130 MC+ 140 0+ Referring to the table:

Unobjectionable mould charge MC+ Objectionable mould charge MC Flash higher than 4 mm i++ Flash from 1 to 2 mm 1+ No flash What is claimed is:

1. A polyester for injection molding prepared by polycondensation of (a) terephthalic acid, (b) ethylene glycol or l,4-bis-hydroxymethylcyclohexane, and (c) an alcohol having more than two primary hydroxyl groups in an amount in the range of from 0.01 to 2% by weight, calculated on the weight of the polyester, said polyester having a reduced specific viscosity in the range of from 0.6 to 2.4 dl/ g. measured with a 1% solution in a 60:40 mixture of phenol and tetrachloroethane at 25 C.

2. The polyester defined in claim 1, wherein the polyester contains, in addition to terephthalic acid units, up to 5% by weight of units of a member selected from the group consisting of isophthalic acid, diphenyl-4,4-dicarboxylic acid, naphthalene-2,6-dicarboxylic acid and adipic acid.

3. The polyester defined in claim 1, wherein component (b) of the polyester composition contains, in addition to ethylene glycol or 1,4-bis-hydroxymethyl-cyclohexane units, up to mol percent of units of a member selected from the group consisting of 2,2-dimethyl-propanediol- (1,3) and butanediol-( 1,4)

4. The polyester defined in claim 1, blended with up to 20% by weight of a copolymer of ethylene and acrylic acid ester or of butadiene and styrene.

5. A shaped article prepared from the composition defined in claim 1.

6. The polyester of claim 1, wherein component (b) of the polyester composition is ethylene glycol.

7. The polyester of claim 1, wherein component (b) of the polyester composition is 1,4-bis-hydroxymethylcyclohexane.

8. The polyester of claim 1, wherein component (c) of the polyester composition is at least one alcohol selected from the group consisting of 1,1,1-tris-hydroxymethyl ethane, 1,1,1-trihydroxymethyl propane, pentaerythritol, dipentaerythritol, l,1,l,6,6,6 hexakis-hydroxymethylhexane, 1,1,3,3-tetrakis hydroxymethyl cyclopentame and l,2,4-trihydroxymethyl-benzene.

9. The polyester of claim 1, wherein the polyester composition has a reduced specific viscosity of from 1.0 to 1.7 dl/g., measured with a 1% solution in a :40 mixture of phenol and tetrachloroethane at 25 C.

References Cited UNITED STATES PATENTS 2,768,213 10/1956 Whetstone 260 2,936,296 5/1960 Precopio et al. 260873 3,360,500 12/1967 Hergenrother 26075 3,402,147 9/1968 Starck et al. 260873 3,446,758 5/1969 Wiener 260--75 3,489,707 1/1970 Fry 260873 FOREIGN PATENTS 6511744 2/ 1967 Netherlands 26075 OTHER REFERENCES Chem. Abst.; vol. 67: 118157t (Sorokin et al.), Heat Polyesters Wire.

Carbide (and Carbon) Chemicals, vol. 33, No. 22, May 30, 1955, p. 2315 (Chemical and Eng. News).

WILLIAM H. \SHORT, Primary Examiner E. WOODBERRY, Assistant Examiner US. Cl. X.R. 

